System for non-invasive controlling of underground storages and method for detecting leakages in underground storages

ABSTRACT

A device and a method is disclosed for detecting an undesired escape of fluids, in that electric potentials are measured using a grid of non-polarizable measuring electrodes above an underground storage facility. Potential differences are determined in such a way that disturbances due to injection borehole or due to the exchange of fluids in the underground storage facility are eliminated. This is done either through the use of the symmetry of the potential distribution around the injection borehole in that the potential differences of pairs of measuring electrodes are observed whose mid-points coincide with the injection borehole or else through the computational simulation of the potentials that arise due to the fluid movement in the underground storage facility without a leak. These are then subtracted from the measured potentials. If the ascertained potential differences exceed a limit value, this is considered as an indication of a leak.

The invention relates to the detection of leaks that can occur in the case of the underground storage of fluids, for example, CO₂, natural gas or compressed air. The invention can be used for all fluids that are stored under pressure in underground storage facilities, natural or man-made voids, which are usually at a depth of 1 km to 3 km. This will be explained here with reference to CO₂ storage by way of example. There are two different types of underground storage facilities, which are also referred to as reservoirs.

I. caverns (artificially created voids) in salt mines and

II. pore storage facilities in which the fissures and pores in the rock are used for storage purposes. On the one hand, depleted gas or oil deposits are used as pore storage facilities and, on the other hand, porous rock (reservoirs) are used in which the water in the pore space is displaced by the injected fluid.

The storage of fluids in underground storage facilities can lead to leaks, as a result of which fluids and, if applicable, saline reservoir water can penetrate into higher groundwater aquifers or even escape from the earth's surface. This endangers the groundwater. In the worst case scenario, a leak can even pose a risk to the health of the population.

The method according to the invention or the device according to the invention is aimed at detecting underground fluid flows. These flows arise in underground storage facilities when a fluid is injected or withdrawn. By the same token, a fluid flow occurs when there is a leak which causes pressurized fluid to escape from the underground storage facility.

Such fluid movements displace water, as a result of which an electric potential arises, usually called a current potential. This potential results from the presence of an electric double layer at the solid-liquid boundary layer in the rock. Due to the fluid flow, part of the electric double layer is carried along and the resultant charge separation causes the macroscopically measurable current potential. In the method according to the invention or with the device according to the invention, this potential can be measured by means of non-polarizable electrodes (measuring electrodes) on the earth's surface or by means of exploratory boreholes.

In case of underground CO₂ storage, seismic measurements are usually used to monitor the underground dissipation of CO₂ (see, for example, Bergmann, p. 2012, Time-lapse seismic and electrical resistivity tomography combined for monitoring of the CO ₂ storage site, Ketzin, Germany, Dissertation, Geoforschungszentrum (Geo-Research Center), Potsdam, Germany). Seismic measurements have the drawback that they are very complex and expensive. Consequently, they cannot be used for continuous monitoring of an underground storage facility.

European patent application EP 595 028 A1 describes a method for monitoring fracking using measuring probes arranged in a grid in order to determine the self potential, whereby the measuring probes are queried cyclically and, to the greatest extent possible, at the same time. This is intended to eliminate the influence of telluric currents. Moreover, it is provided that, aside from the self potential, the earth resistivity between the measuring probes and the base probe is also determined cyclically. Therefore, the invention makes it possible to determine the potentials on the surface of the region that is to be examined, independently of changes in the earth resistivity. The potentials thus determined provide an indication of the strength and course of underground disturbances that are caused, for example, by fracking.

German patent application DE 35 29 466 A1 describes a method for the determination of the boundaries of underground deposits of natural gas. Here, at least one self potential probe profile from several probes is created. The probes are placed on the earth's surface in such a way that several measuring probes are on the outside of the earth's surface area covering the deposit and several measuring probes are on the inside. The self potentials of all of the probes are queried at periodical measuring cycles and they are filtered in such a way that the higher-frequency measured value changes are separated from the considerably longer periodical measured value changes. On the basis of the higher-frequency measured value changes of each probe, a mean value and/or a cumulative value is determined as the measure of the higher-frequency amplitudes of the associated probe. The higher-frequency amplitudes of the probes of a measuring probe profile are compared to each other, and the place of an abrupt change in the higher-frequency amplitudes of adjacent probes is considered as being the boundary of the underground deposit.

The objective of the invention is to put forward a method and a device for detecting the spread of fluids underground, whereby the device and the method are cost-effective, they can be used on a permanent basis, and they recognize leaks.

The term “permanent” refers to a period of several weeks up to five years or longer. In case of CO₂ storage, CO₂ moves underground during the injection period and displaces water. The water movement generates a voltage signal. This continues until the injected fluid (here CO₂) has reached a locally stable state in the underground storage facility. The occurrence of leaks is most probable during this period of time. Leaks that occur after this time can be detected as long as self potential measurements are carried out.

The objective of the invention is achieved by the features of the method claim 1 and of the device claim 9. Additional advantageous refinements are described in the subordinate claims.

The electric potential is supposed to be detected here using a not necessarily equidistant grid of measuring probes that are on or under the earth's surface (in exploratory boreholes). The measuring probes cover the area of the underground storage facility, whereby probes are located above as well as outside of the area covering the underground storage facility. The potential differences between the measuring probes and a non-polarizable base probe are queried cyclically. In this context, the term “cyclically” means that the potential differences are queried at prescribed time intervals over a prescribed period of time at a prescribed frequency during the prescribed period of time. The direct voltage component is extracted on the basis of the measured data and subsequently analyzed.

Leaks that occur in an underground storage facility can be detected on the basis of elevated potential values above the leak, as long as the fluid movement caused by the leak is not too deep.

If leaks occur while fluid is being fed into or withdrawn from underground storage facilities, the potential that arises in the storage facility due to the fluid flow is superimposed onto the potential caused by the leak. Underground storage facilities are normally at such a great depth (>1000 m) that the current potential that arises there would be below the detection limit on the earth's surface if the conductive piping of the injection borehole did not electrically connect the underground storage facility to the earth's surface. Only due to this electric connection can potentials that arise in the underground storage facility be detected on the earth's surface. Therefore, within the scope of this invention, the term “injection borehole” encompasses the piping.

Due to the metal piping as well as the high pressures at the injection and withdrawal sites, the current potential that arose in the underground storage facility is superimposed onto the potential caused by the leak, as a result of which a direct detection of the leak on the basis of the potential distribution is not possible.

Moreover, the metal piping can also cause redox potentials that arise due to reduction and oxidation reactions at the metal piping of the borehole. These redox potentials are many times higher than the current potentials caused by leaks, thus preventing their direct detection.

The method according to the invention calls for detecting or computing potential differences in such a way that interferences such as redox potentials and current potentials are eliminated, as a result of which leaks can be detected.

This can be done in two ways:

-   I. In one method step, the symmetry of the potential distribution is     used and the potential differences are observed between     non-polarizable measuring electrodes that are arranged symmetrically     relative to the injection site, for example, that are arranged at     equal distances from the injection site. In the case of an isotropic     underground storage facility (also referred to as a pore storage     facility), the potentials caused by fluid injection or fluid     withdrawal are identical at the same distance (radius) symmetrically     relative to the injection site, as a result of which its influence     is eliminated by difference formation. -   II. In another method step, a potential difference is determined     between a measured potential and an anticipated potential without a     leak or disturbance, at each measuring point of the non-polarizable     measuring electrodes. If the underground storage facility has a     heterogeneous permeability or geometry, the current potential that     is simulated applies to the underground storage facility without a     leak, said current potential arising from fluid injection or fluid     withdrawal and subsequently being subtracted from the measured     potential. The SHEMAT (Simulator of HEat and MAss Transport) Suite     or Comsol, for instance, can be used for the computational     simulation. Such a simulation can also be used in the case of an     isotropic underground storage facility. Here, the redox potential is     assumed to be constant over time and it has to be acquired before     the start-up of the underground storage facility.

These two above-mentioned types of method steps for carrying out the method according to the invention can be carried out alternatively to each other or together in parallel with each other, depending on the embodiment. In one embodiment of the method according to the invention, method step (I) (type I) or method step (II) (type II) is carried out. In another embodiment, the method according to the invention encompasses both of the above-mentioned method steps.

If there is a leak, elevated potential differences occur above the leak in both cases. Consequently, the method makes it possible to also recognize weaker potentials stemming from a fluid flow at an unexpected place such as a leak.

Advantageous embodiments and refinements of the method are described below which can disclose likewise inventive aspects either individually or else in combination.

The distance that is to be selected between the non-polarizable electrodes relative to each other depends on the depth in which the fluid flows are supposed to be detected. If the rising fluid is supposed to be detected directly, then an approximation “electric point source” (in this context, also see FIG. 5) can be used to compute the electric potential on the earth's surface as a function of the source depth. Here, the source depth corresponds to the upper end of the rising fluid. If 1 mV is assumed as the detection limit for potentials, then a point source can be detected down to a depth of about 200 meters, whereby the detection depth also depends on the electric conductivity of the rock and on the source strength, which is why the detection depth can be considerably less for low source strengths or for a very conductive underground.

In order to be able to reliably detect the potential anomalies caused by leaks, the selected electrode distance should not be greater than 100 meters. If the fluid that has escaped from the underground storage facility spreads in a groundwater aquifer that is situated above it, then the potential anomaly (also see FIGS. 3a and 3b ) spreads in a time-dependent manner, so that it is detected in any case if the groundwater aquifer is not situated too deep.

In a preferred embodiment, it is provided that a plurality of measuring electrodes is arranged on the earth's surface and/or in exploratory boreholes so as to cover the area of the underground storage facility.

In a preferred embodiment, if a prescribed potential difference limit value is exceeded, an interference relative to the measuring point of the measuring electrode can be detected, whereby the interference can then be displayed.

The invention also relates to a device for carrying out the method according to the invention, involving an underground storage facility and an injection borehole (also referred to as a feed borehole) leading to the underground storage facility, whereby measuring electrodes are arranged symmetrically around the injection borehole leading to the underground storage facility.

The device is suitable for carrying out a method for detecting underground fluid flows. The device comprises measuring electrodes that, in one embodiment, are arranged so as to cover the area of the underground storage facility, whereby the device comprises a module for providing potential reference values at the positions of the measuring electrodes.

It has proven to be advantageous for the module to comprise a simulation module to compute potential reference values for the leak-free case for purposes of the difference formation, taking into account the momentary inflow of fluid, the geometry and the physical properties of the underground storage facility, especially the permeability, the porosity, the specific electric conductivity and the coupling coefficient.

In one embodiment of the device, a plurality of measuring electrodes is arranged on the earth's surface and/or in exploratory boreholes so as to cover the area of the underground storage facility.

In another embodiment of the device, the measuring electrodes are arranged at a distance of not more than 100 meters from each other.

Below, the invention will be explained in greater depth with reference to the embodiments shown in schematic form in the drawings. These embodiments are merely conceivable embodiments and should not be construed in a limiting fashion.

BRIEF DESCRIPTION OF THE FIGURES

These and other aspects of the invention are shown in detail in the figures as follows:

FIG. 1a section through the earth layers in a three-dimensional view, comprising an underground storage facility with a leak and a feed line or injection borehole;

FIG. 2a a section through the earth layers after 1000 days of CO₂ injection, showing an underground storage facility and a groundwater aquifer situated above it into which CO₂ has penetrated due to a leak;

FIG. 2b the measuring set-up of dipole arrangements that are symmetrical relative to the injection borehole;

FIG. 3a by way of an example, the determination of a leak by means of the simulation of potential reference values; the difference between the potentials on the surface with the leak is shown;

FIG. 3b by way of an example, a measurement (potential difference) with a measuring set-up according to FIG. 2 b;

FIG. 4 the schematic sequence of the method and of the device;

FIG. 5 electric potential on the earth's surface that is caused by a point source in the depth z (in meters).

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1 shows a schematic view of the structure of an underground section 2 with several earth layers 3 under the earth's surface 1, as well as the storage of CO₂ in an underground storage facility 21 that has a leak 25. CO₂ is fed into the underground storage facility 21 via a feed borehole 23, also referred to as an injection borehole 23. A cover layer 22 that is impermeable to CO₂ is above the underground storage facility 21. This cover layer 22 has a defect due to which a leak 25 occurs. Such defects can arise, for example, due to faults or abandoned boreholes. Due to the leak 25, CO₂ and saline reservoir water penetrate into a groundwater aquifer 5. The CO₂ displaces the water there, thus forming a CO₂ accumulation 27. Water is removed from the groundwater aquifer 5 at a withdrawal site 7.

By way of example, FIG. 2a shows a section through the underground section 2 after 1000 days of CO₂ injection at an injection rate of 1.25 kg per second. This figure also shows the CO₂ accumulation 9 in the underground storage facility 21 and in a groundwater aquifer above it into which CO₂ has penetrated as an accumulation 27 because of the leak 25 (old borehole with a diameter of 36 cm, not shown here).

FIG. 2b shows, for instance, a CO₂ accumulation 9 in the underground storage facility 21 after 1000 days of CO₂ injection at an injection rate of 1.25 kg per second without a leak. Non-polarizable measuring electrodes 11, 31, 19, also referred to a measuring probes, are arranged symmetrically around the injection borehole 23 in a dipole arrangement 15.

The measuring electrodes 11 on one side are referred to as first measuring electrodes 13 with which, in this embodiment, a second measuring electrode 19 is associated symmetrically on the other side of the injection borehole 23 and between which the potential difference is then computed. The mid-point of each dipole arrangement made up of a first measuring electrode 13 and a second measuring electrode 19 coincides on the earth's surface 1 with the position of the injection borehole/feed borehole 23 leading to the underground storage facility 21. The measuring electrodes 11 can be configured identically. In particular, Pb/PbCl₂, Cu/CuSO₄ and Ag/AgCl can be used as the measuring electrodes 11.

By way of example, FIG. 3a shows a depiction of the location-dependent potential difference after 1000 days of CO₂ injection (wherein x=815 m) at an injection rate of 1.25 kg per second with a leak (wherein x=915 meters) that is obtained with a measuring set-up according to FIG. 2b , whereby the determined potential difference is plotted at the site of the measuring electrode 13. Without a leak, the potential difference would be zero everywhere.

By way of example, FIG. 3b shows a measured curve after 1000 days of CO₂ injection, whereby the potential difference between the simulation of potential values and the measured potential values with a leak is shown. The injection (wherein x=815 m) was carried out at an injection rate of 1.25 kg per second. The leak is at approximately x=950 meters.

FIG. 4 shows a flow chart on the basis of which a method for detecting a leak in conjunction with the storage of fluids in underground storage facilities 21 is explained in greater detail. A central control unit 31 controls the simultaneous acquisition of potential measured values from all of the measuring electrodes 11 by means of a device 41 at prescribed points in time. The simultaneous query of the potential values from all of the measuring electrodes 11 has the advantage that interferences that vary over time but are the same at all of the measuring electrodes and that are due to, for instance, telluric flows or temperature fluctuations, especially during the computation of the potential difference between the electrodes that are arranged symmetrically relative to the injection borehole, drop out and consequently do not influence the result.

The measured potentials are further processed in the evaluation module 49. Here, in the case of an isotropic underground storage facility, the potential differences of the electrodes that are at the same distance from the injection borehole, as shown in FIG. 2b , are computed. No simulation is needed for this purpose. If the underground storage facility is not isotropic or if the measuring electrodes are not arranged symmetrically relative to the injection borehole, then the potential for the case without a leak is simulated by a module 47, also referred to as a simulation module 47, for all of the measuring points in time and for all of the positions of the measuring electrodes 11. The simulation module 47 comprises a processor for carrying out the simulations. The simulated potential is subsequently subtracted from the measured potentials in the evaluation module 49. It can also be provided that potential values for the case without a leak are stored in the simulation module 47 for the different measuring points and times, and they are only queried.

The potential differences computed in the evaluation module 49 are transferred to a module 51 in order to recognize when a permissible difference value has been exceeded. Permissible limit values for all of the measuring electrodes 11 are stored in the module 51. It can also be provided that and identical limit value is stored for all of the measuring electrodes 11. If the prescribed limit value is exceeded, this information is forwarded to a signal generator 53. The signal generator 53 then provides information about the fact that the limit value has been exceeded, for instance, by automatically sending messages to a monitoring central system.

The embodiments shown here constitute merely examples of the present invention and therefore must not be construed in a limiting fashion. Alternative embodiments considered by the person skilled in the art are likewise encompassed by the scope of protection of the present invention.

LIST OF REFERENCE NUMERALS

-   1 earth's surface -   2 underground section -   3 earth layers -   5 groundwater aquifer -   7 groundwater withdrawal site -   9 CO₂ accumulation -   11 measuring electrode -   13 first measuring electrode -   14 measuring point -   15 dipole arrangement -   17 mid-point of the dipole arrangement -   19 second measuring electrode -   21 underground storage facility -   22 cover layer -   23 feed borehole/injection borehole -   25 leak site -   27 CO₂ accumulation due to leak -   31 central control unit -   41 device for providing the potential at a measuring point -   47 simulation module -   49 evaluation module -   51 module for recognizing when a permissible limit value has been     exceeded -   53 signal generator 

1. A method for detecting underground fluid flows, whereby a fluid can be fed into an underground storage facility through an injection borehole, and potential values are picked up by non-polarizable measuring electrodes, whereby the method comprises at least one of the following steps: the symmetry of the potential distribution around the injection borehole is used in order to form potential differences between the measured values of non-polarizable measuring electrodes that are arranged symmetrically to the injection borehole, and/or a potential difference is determined between a measured potential and an anticipated potential without a leak or disturbance, at each measuring point of the non-polarizable measuring electrodes.
 2. The method according to claim 1, characterized in that in the case of an isotropic structure of the underground storage facility, the potential difference between the measuring electrodes is determined, their mid-point coincides with the injection borehole, whereby the potential values are preferably picked up at the two measuring electrodes at the same time.
 3. The method according to claim 2, characterized in that if a prescribed potential difference limit value is exceeded, an interference is recognized that is present at the measuring point of the first measuring electrode or at the measuring point of the second measuring electrode, as a function of the sign of the measured potential difference.
 4. The method according to claim 1, characterized in that if the underground storage facility has a heterogeneous permeability or geometry, a potential reference value for the difference formation is based on a computational simulation of the fluid flow in the underground storage facility without disturbances, especially without a leak.
 5. The method according to claim 4, characterized in that if a predetermined potential difference limit value is exceeded, an interference pertaining to the measuring point of the measuring electrode is recognized.
 6. The method according to one of the preceding claims claim 1, characterized in that a plurality of measuring electrodes that can be used as first and second measuring electrodes is arranged on the earth's surface and/or in exploratory boreholes so as to cover the area of the underground storage facility.
 7. The method according to claim 6, characterized in that the measuring electrodes are arranged at a distance of not more than 100 m from each other.
 8. A device for carrying out a method according to claim 1, involving an underground storage facility and an injection borehole leading to the underground storage facility, whereby measuring electrodes are arranged symmetrically around the injection borehole leading to the underground storage facility.
 9. The device according to claim 8, characterized in that measuring electrodes are arranged so as to cover the area of the underground storage facility, whereby the device comprises a module for determining the potential differences at the individual measuring points of the measuring electrodes.
 10. The device according to claim 9, characterized in that the device has a simulation module for computing a potential reference value for purposes of the difference formation, taking into account the momentary inflow of fluid, the geometry and the physical properties of the underground storage facility, especially the permeability, the porosity, the specific electric conductivity and the coupling coefficient.
 11. The device according to claim 8, characterized in that a plurality of measuring electrodes is arranged on the earth's surface and/or in exploratory boreholes so as to cover the area of the underground storage facility.
 12. The device according to claim 11, characterized in that the measuring electrodes are arranged at a distance of no more than 100 m from each other. 